1. FIELD OF THE INVENTION
The invention relates to the separation of hydrocarbons through the use of adsorptive separation in which a feed stream containing an admixture of hydrocarbons is contacted with a solid adsorbent which selectively reatins one or more of the hydrocarbons. The invention specifically relates to the solid bed adsorptive separation of 2,6 dimethylnaphthalene ["2,6 DMN"]from a feed stream comprising a mixture of dimethylnaphthalene ["DMN"]isomers. More specifically, the invention relates to an adsorptive separation process for purifying 2,6 DMN from a feed mixture comprising DMN isomers by emplying an appropriate adsorbent/desorbent system operated in a two-stage configuration.
2. BACKGROUND INFORMATION
High purity 2,6 DMN is an important intermediate material to the production of 2,6 Naphthalenedicarboxylic acid ["2,6 NDCA"]. Moreover, polymers derived from 2,6 NDCA are known to possess properties that make them more desirable in certain applications compred to other polymers such as those derived from terephthalic acid. Such preferred polymers include those such as polyesters, polyamides, and polyaramides.
It is commercially possible to obtain 2,6 DMN from certain common industrial sources, namely, heavy catalytic reformate, fluid catalytic cracking process recycle oil or coal tar. Alternatively, it is possible to synthesize an isomeric mixture of DMN's.
Regardless of the source of the 2,6 DMN, such material is invariably found in conjunction with an isomeric mixture of dimethylnaphthalenes and other impurities which make the purification of 2,6 DMN a less than simple matter. Particularly difficult is the separation of the mixture of the 2,6 DMN and 2,7 DMN isomers which, during the purification operation, form a binary eutectic mixture of approximately 42% 2,6 DMN and 58% 2,7 DMN. This eutectic cannot be broken by distillation or solvent crystallization.
Current methods of obtaining high purity 2,6 DMN involve the use of sequential unit operations such as adsorptive separation followed by crystallization and/or complexing reactions to achieve a high purity 2,6 DMN product.
For example, Hedge teaches, in U.S. Pat. No. 3,668,267, that an adsorptive separation process, using a sodium-exchanged, type Y zeolite adsorbent in conjunction with a subsequent crystallization step, can be used to obtain acceptable pure 2,6 DMN. In such case, the adsorptive step selectively rejected 2,6 DMN to a raffinate stream which stream was, in turn, used as the feed to the crystallization stage. Hedge also disclosed the capability of a type L-zeolite to selectively adsorb the 2,6 DMN isomer from a DMN feed mixture, however, the aforesaid two-stage process (i.e., adsorptive separation followed by crystallization) was disclosed to produce a 2,6 DMN product of superior quality.
Subsequently, Hedge, in U.S. Pat. No. 3,772,399 teaches a method of separating 2,6 DMN from a mixture containing 2,6 DMN and 1,5 DMN, using a partially dehydrated type L zeolite adsorbent.
Japanese Disclosure No. 240632/87 is believed pertinent to the extent that therein is taught the tendency of 2,6 DMN to be more strongly adsorbed onto a potassium-exchanged type X zeolite adsorbent, relative to the 1,4 DMN isomer. To the contrary, we have discovered that such adsorbent, when used with either a toluene or monochlorobenzene desorbent material, always exhibits a tendency to reject the 2,6 DMN isomer relative to the other DMN isomers.
It is believed pertinent that the prior art adsorption processes have been largely concerned with other than 2,6 DMN adsorptive operations and that the final purification of 2,6 DMN is accomplished in the prior art by non-adsorptive means after the adsorptive removal of the other eutectic constituent, e.g. 2,7 DMN.